Method and apparatus for performing fast power control in a mobile communication system

ABSTRACT

A method and apparatus for controlling transmission power levels in a mobile communication system. The method provides for a closed-loop power control method. A mobile station provides information on the quality of the signal received from the base station, and the base station responds by adjusting the power allocated to that user in a shared base station signal. The transmission power is adjusted initially by a large increment and then ramped down at an increasingly decreasing rate. The mobile station also provides information to the base station as to its relative velocity and the base station adjusts its transmission power in accordance with this velocity information.

CROSS REFERENCE INFORMATION

[0001] This application is a continuation application of applicationSer. No. 09/454,926, filed Dec. 3, 1999 which is a continuationapplication of Ser. No. 08/958,882, filed Oct. 27, 1997, U.S. Pat. No.6,035,209, which is a file wrapper continuation application of Ser. No.08/414,633, filed Mar. 31, 1995, now abandoned.

BACKGROUND OF THE INVENTION

[0002] I. Field of the Invention

[0003] The present invention relates to communication systems. Moreparticularly, the present invention relates to a novel and improvedmethod and apparatus for controlling transmission power in a mobilecommunication system.

[0004] II. Description of the Related Art

[0005] The use of code division multiple access (CDMA) modulationtechniques is one of several techniques for facilitating communicationsin which a large number of system users are present. Other multipleaccess communication system techniques, such as time division multipleaccess (TDMA) and frequency division multiple access (FDMA) are known inthe art. However, the spread spectrum modulation technique of CDMA hassignificant advantages over these modulation techniques for multipleaccess communication systems. The use of CDMA techniques in a multipleaccess communication system is disclosed in U.S. Pat. No. 4,901,307,entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USINGSATELLITE OR TERRESTRIAL REPEATERS”, assigned to the assignee of thepresent invention, of which the disclosure thereof is incorporated byreference herein. The use of CDMA techniques in a multiple accesscommunication system is further disclosed in U.S. Pat. No. 5,103,459,entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMACELLULAR TELEPHONE SYSTEM”, assigned to the assignee of the presentinvention, of which the disclosure thereof is incorporated by referenceherein.

[0006] CDMA by its inherent nature of being a wideband signal offers aform of frequency diversity by spreading the signal energy over a widebandwidth. Therefore, frequency selective fading affects only a smallpart of the CDMA signal bandwidth. Space or path diversity is obtainedby providing multiple signal paths through simultaneous links from amobile user through two or more cell-sites. Furthermore, path diversitymay be obtained by exploiting the multipath environment through spreadspectrum processing by allowing a signal arriving with differentpropagation delays to be received and processed separately. Examples ofpath diversity are illustrated in U.S. Pat. No. 5,101,501 entitled“METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN ACDMA CELLULAR TELEPHONE SYSTEM”, and U.S. Pat. No. 5,109,390 entitled“DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM”, both assignedto the assignee of the present invention and incorporated by referenceherein.

[0007] A method for transmission of speech in digital communicationsystems that offers particular advantages in increasing capacity whilemaintaining high quality of perceived speech is by the use of variablerate speech encoding. The method and apparatus of a particularly usefulvariable rate speech encoder is described in detail in copending U.S.Pat. No. 5,414,796 which is a continuation application of U.S. patentapplication Ser. No. 07/713,661, filed Jun. 11, 1991, entitled “VARIABLERATE VOCODER”, assigned to the assignee of the present invention, ofwhich the disclosure thereof is incorporated by reference herein.

[0008] The use of a variable rate speech encoder provides for dataframes of maximum speech data capacity when said speech encoding isproviding speech data at a maximum rate. When a variable rate speechcoder is providing speech data at a less that maximum rate, there isexcess capacity in the transmission frames. A method for transmittingadditional data in transmission frames of a fixed predetermined size,wherein the source of the data for the data frames is providing the dataat a variable rate is described in detail in copending U.S. Pat. No.5,504,773, which is a continuation application of U.S. patentapplication Ser. No. 07/822,164, filed Jan. 16, 1992, entitled “METHODAND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION”, assigned tothe assignee of the present invention, of which the disclosure thereofis incorporated by reference herein. In the above mentioned patentapplication a method and apparatus is disclosed for combining data ofdiffering types from different sources in a data frame for transmission.

[0009] In frames containing less data than a predetermined capacity,power consumption may be lessened by transmission gating a transmissionamplifier such that only parts of the frame containing data aretransmitted. Furthermore, message collisions in a communication systemmay be reduced if the data is placed into frames in accordance with apredetermined pseudorandom process. A method and apparatus for gatingthe transmission and for positioning the data in the frames is disclosedin U.S. Pat. No. 5,659,569, which is a continuation application of U.S.patent application Ser. No. 07/846,312, filed Mar. 5, 1992, entitled“DATA BURST RANDOMIZER”, assigned to the assignee of the presentinvention, of which the disclosure thereof is incorporated by referenceherein.

[0010] A useful method of power control of a mobile in a communicationsystem is to monitor the power of the received signal from the mobilestation at a base station. The base station in response to the monitoredpower level transmits power control bits to the mobile station atregular intervals. A method and apparatus for controlling transmissionpower in this fashion is disclosed in U.S. Pat. No. 5,056,109, entitled“METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMACELLULAR TELEPHONE SYSTEM”, assigned to the assignee of the presentinvention, of which the disclosure thereof is incorporated by referenceherein.

[0011] In a communication system that provides data using a QPSKmodulation format, very useful information can be obtained by taking thecross product of the I and Q components of the QPSK signal. By knowingthe relative phases of the two components, one can determine roughly thevelocity of the mobile station in relation to the base station. Adescription of a circuit for determining the cross product of the I andQ components in a QPSK modulation communication system is disclosed inU.S. Pat. No. 5,506,865, entitled “PILOT CARRIER DOT PRODUCT CIRCUIT”,assigned to the assignee of the present invention, the disclosure ofwhich is incorporated by reference herein.

[0012] In an alternative continuous transmission strategy, if the datarate is less than the predetermined maximum the data is repeated withinthe frame such that the data occupies the full capacity of the dataframe. If such a strategy is employed, power consumption andinterference to other users may be reduced during periods of datatransmission at less than the predetermined maximum by reducing thepower at which the frame is transmitted. This reduced transmission poweris compensated by the redundancy in the data stream and can offerbenefits in range for a fixed maximum transmission power.

[0013] A problem that is encountered in controlling transmission powerin the continuous transmission strategy is that the receiver does notknow the transmission rate a priori and so does not know the power levelthat should be received. The present invention provides a method andapparatus for controlling transmission power in a continuoustransmission communication system.

SUMMARY OF THE INVENTION

[0014] The present invention is a novel and improved method andapparatus for closed loop transmission power control in a communicationsystem. It is an object of the present invention to provide timely powercontrol that is necessary to provide robust communication link qualityunder fading conditions.

[0015] Further, it should be noted that power control techniques arepresented in the exemplary embodiment in a spread spectrum communicationsystem, however, the methods presented are equally applicable for othercommunication systems. Also, the exemplary embodiment used for thecontrol of transmission power in transmissions from a base station to aremote or mobile station may be applied to the control of transmissionpower in transmissions from a remote or mobile station to a basestation.

[0016] In the exemplary embodiment, a base station transmits packets ofdata to a mobile station. The mobile station receives, demodulates anddecodes the received packet. If the mobile station determines that thereceived packet cannot be reliably decoded, it sets the normally ‘0’quality response power control bit to ‘1’ indicating the situation tothe base station. In response, the base station increases thetransmission power of the signal to the mobile station.

[0017] In the exemplary embodiment of the present invention, when thebase station increase its transmission power it does so with arelatively large step in transmission power which is assumed to be morethan adequate under most fading conditions. The base station thendecreases the transmission power level at an exponentially decreasingrate as long as the quality response power control bits remain at ‘0’.In an alternative embodiment, the base station responds to a requestfrom the mobile station for additional signal power by increasing thesignal power incrementally.

[0018] In an improved embodiment of this power control system, the basestation will determine whether the error reported by the mobile stationwas of a random nature in which case it will immediately begin rampingdown the transmission power or whether the error was an error resultingfrom a genuine fading condition. The base station distinguishes errorsof a random nature from those of a prolonged nature by examining thepatterns of power control bits sent by the mobile station. If thepattern of power control request signals and sends a 1-bit quality powercontrol response in the packets it transmits back to the base stationand indicates that there is a new fading condition present in thepropagation path, then the base station will refrain from decreasing thetransmission power.

[0019] One of the identified sources of sudden changes in thepropagation path of a mobile station is a change in velocity relative tothe position of the base station. That is, if the velocity towards themobile station or away from the mobile station is changing. In thepresent invention, the mobile station determines that the velocityrelative to the base station is changing, and if necessary, sets thepower control bits to request additional power from the base station toaccommodate the change in velocity.

[0020] In a first exemplary embodiment, the mobile station is equippedwith a motion sensor which may operate off of information from thespeedometer or tachometer in the case of an automobile based mobilestation. The mobile station then generates the power control signal inaccordance with the signal from the motion sensor.

[0021] In a second exemplary embodiment, the mobile station may sense ashift in the received signal from the base station in order to sensemotion. In the exemplary embodiment, the mobile station determines thechanges in relative velocity by measuring the Doppler shift in thereceived pilot signal.

[0022] In a third exemplary embodiment, the base station determines thepresence of motion by sensing changes in the incoming signal and adjuststransmission power in accordance with these changes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The features, objects, and advantages of the present inventionwill become more apparent from the detailed description set forth belowwhen taken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

[0024]FIG. 1 is an illustration of an exemplary mobile telephone system;

[0025]FIG. 2 is an illustration of the apparatus of the presentinvention; and

[0026]FIG. 3 is an illustration of a curve illustrating the delay timeentailed in a closed loop power control system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] Referring to FIG. 1, the present invention is illustrated in anexemplary implementation in a mobile communication system forcontrolling the power of transmissions between base station 4 and mobilestation 6. Information may be provided to and from a public switchedtelephone network (PSTN) to system controller and switch 2, or may beprovided to and from controller and switch 2 by another base station ifthe call is a mobile station to mobile station communication. Systemcontroller and switch 2, in turn, provides data to and receives datafrom base station 4. Base station 4 transmits data to and receives datafrom mobile station 6.

[0028] In the exemplary embodiment the signals transmitted between basestation 4 and mobile station 6 are spread spectrum communicationsignals, the generation of the waveforms of which are described indetail in the above mentioned U.S. Pat. No. 4,901,307 and U.S. Pat. No.5,103,459. The transmission link for communication of messages betweenmobile station 6 and base station 4 is referred to as the reverse linkand the transmission link for communication of messages between basestation 4 and mobile station 6 is referred to as the forward link. Inthe exemplary embodiment, the present invention is used to control thetransmission power of base station 4. However, the methods of powercontrol of the present invention are equally applicable to controllingthe transmission power of mobile station 6.

[0029] Referring to FIG. 2, base station 50 and mobile station 30 areillustrated in block diagram form showing the apparatus for providingcontrol of the transmission power of base station 50 of the presentinvention. If a communication link degrades, then the link quality canbe improved by increasing the transmission power of the transmittingdevice. In the exemplary embodiment of controlling transmission power ofbase station 50, some of the methods for determining that thetransmission power of base station 50 should be increased include:

[0030] (a) mobile station detection of frame errors on forward link;

[0031] (b) mobile station detects that received power is low on forwardlink;

[0032] (c) mobile station to base station range is large;

[0033] (d) mobile station location is poor;

[0034] (e) mobile station change in velocity; and

[0035] (f) mobile station detects received power on pilot channel is lowon forward link.

[0036] Conversely, some of the methods for determining that thetransmission power of base station 50 should be decreased include:

[0037] (a) mobile station quality responses to the base station show alow frame error rate for the forward link;

[0038] (b) mobile station detects that received power is high on forwardlink;

[0039] (c) base station to mobile station range is low;

[0040] (d) mobile station location is good; and

[0041] (e) mobile station detects that received power on forward linkpilot channel is high.

[0042] When base station 50 detects a need to modify the transmissionpower of the forward link, control processor 58 sends a signalspecifying a modified transmission power to transmitter (TMTR) 64. Themodified power signal may simply indicate a need to increase or decreasethe transmission power or it may indicate an amount to change the signalpower or it may be an absolute signal power level. In response to themodified power level signal, transmitter 64 provides all transmission atthe modified transmission power level.

[0043] It should be noted that data source 60 may source modem,facsimile or speech data. Data source 60 may be a variable rate sourcethat varies its transmission rate on a frame to frame basis throughoutthe transmission or may be able to vary rates only upon command. In theexemplary embodiment, data source 60 is a variable rate vocoder. Thedesign and implementation of a variable rate speech vocoder aredescribed in detail in the aforementioned application Ser. No.08/004,484. The output from data source 60 is encoded by encoder 62 andinput to traffic modulator 63 for modulation and input to transmitter64. Also input to transmitter 65 is a synchronous pilot signal fortransmission.

[0044] A need for modification of the transmission power may beindicated by any one of the conditions enumerated above or by anycombination of those conditions. If the method of power control is basedupon a position related effect such as range or mobile station location,then an external signal (LOCATION) is provided to control processor 58of base station 50 indicative of the location condition. The rangecondition can be detected by base station 50. In an alternativeembodiment the range condition can be detected by mobile station 30 andtransmitted to base station 50. In response to the detected rangecondition control processor 58 in base station 50 generates a controlsignal for modifying transmission power of transmitter 64.

[0045] In a closed loop power control implementation, power controlsignals are provided from mobile station 30 to base station 50. Mobilestation 30 may determine the power control signal in accordance withreceived power or alternatively in accordance with the detection offrame errors. The present invention is equally applicable to any linkquality factors.

[0046] If the link quality factor used is received power, then thesignal from base station 50 received at mobile station 30 by antenna 38and provided to receiver (RCVR) 42 which provides an indication of thereceived power to control processor 46. If the link quality factor usedis the detection of frame errors, then receiver 42 downconverts andamplifies the signal providing the received signal to trafficdemodulator 43. If the traffic signal is accompanied by a pilot signalin order to provide for coherent demodulation then the received signalis also provided to pilot demodulator 45 which demodulates the signal inaccordance with a pilot demodulation format and provides a timing signalto traffic demodulator 43. Traffic demodulator 43 demodulates thereceived signal in accordance with a traffic demodulator format. In theexemplary embodiment, traffic demodulator 43 and pilot demodulator 45are CDMA spread spectrum demodulators, the design of which is describedin the aforementioned U.S. Pat. Nos. 4,901,307 and 5,103,459. Trafficdemodulator 43 provides the demodulated signal to decoder 44. In a firstexemplary embodiment, decoder 44 performs error—detection decoding todetermine if errors have occurred. Error detection/correction decoderssuch as the Viterbi trellis decoder are well known in the art. In analternative embodiment, decoder 44 decodes the demodulated signal andthen re-encodes the decoded signal. Decoder 44 then compares there-encoded signal with the demodulated signal to obtain an estimate ofthe channel symbol error rate. Decoder 44 provides a signal indicatingan estimated channel symbol error rate to control processor 46.

[0047] Control processor 46 compares the received power or estimatedchannel symbol error rate referred to generically as the link qualityfactor against a threshold or set of thresholds which may be static orvarying. Control processor 46, then provides the power controlinformation to either encoder 34 or power control encoder (P.C. ENC.)47. If the power control information is to be encoded into the dataframe, then the power control data is provided to encoder 34. Thismethod requires that an entire frame of data be processed beforetransmitting the power control data, then encoded traffic datacontaining power control data are provided to transmitter (TMTR) 36through modulator 35. In an alternative embodiment, the power controldata may simply overwrite portions of the data frame or may be placed inpredetermined vacant positions in the transmission frame. If the powercontrol data overwrites traffic data, then this may be corrected byforward error correction techniques at base station 50.

[0048] In implementations that process a full frame of data beforeproviding the power control data, the delay waiting for a full frame tobe processed is undesirable in fast fade conditions. The alternative isto provide the power control data directly to modulator 35 where it maybe punctured into the outgoing data stream. If the power control data istransmitted without error correction coding then control processor 46outputs the power control data directly to modulator 35. If errorcorrection coding is desired for the power control data, controlprocessor 46 outputs the power control data to power control encoder 47which encodes power control data without regard to the outgoing trafficdata. Power control encoder 47 provides the encoded power control signalto modulator 35 which combines the encoded power control signal with theoutgoing traffic data provided from data source 32 through encoder 34 tomodulator 35. Transmitter 36 upconverts and amplifies the signal andprovides it to antenna 38 for transmission to base station 50.

[0049] The transmitted signal is received at antenna 52 of base station50 and provided to data receiver (RCVR) 54 where it is downconverted andamplified. Receiver 54 provides the received signal to demodulator 55which demodulates the received signal. In the exemplary embodiment,demodulator 55 is a CDMA spread spectrum demodulator which is describedin detail in the aforementioned U.S. Pat. Nos. 4,901,307 and 5,103,459.If the power control data is encoded within a frame of traffic data,then the traffic and power control data is provided to decoder 56.Decoder 56 decodes the signal and separates the power control signalfrom the traffic data.

[0050] If, on the other hand the power control data is not encoded witha full frame of data but rather punctured into the transmission streamof data, then demodulator 55 demodulates the signal and extracts thepower control data from the incoming data stream. If the power controlsignal is not encoded then demodulator 55 provides the power controldata directly to control processor 58. If the power control signal isencoded then demodulator 55 provides the encoded power control data topower control decoder (P.C. DEC.) 55. Power control decoder 55 decodesthe power control data and provides the decoded power control data tocontrol processor 58. The power control signal is provided to controlprocessor 58, which in accordance with the power control signal providesa control signal to transmitter 64 indicative of a modified transmissionpower level.

[0051] One of the inherent problems with closed-loop power controlsystems is a relatively slow response time, relative to an open-looppower control system. For example, in a closed-loop power controlsystem, when base station 50 transmits a frame at an insufficienttransmission energy to mobile station 30, mobile station 30 receives anddecodes the frame, determines whether the frame is in error, prepares apower control message indicating the frame error, then transmits thepower control message to base station 50, which decodes the frame,extracts the power control message and adjusts the transmission power oftransmitter 64. This results in a four frame time log before correctionis apparent at mobile station 30. Thus, if the propagation path hasdeteriorated, four consecutive frames would be transmitted at the sameinsufficient frame energy before a frame is transmitted at the adjustedframe energy. In this delay period the fading condition may havesubstantially improved or deteriorated.

[0052] The following are methods by which to improve the responsivenessof a closed power control system. In a first exemplary embodiment of thepresent invention, the base station assumes the worse case. That is thatthe propagation path has deteriorated during the four frame delayperiod. In response the base station increases the transmission energyto that user by a relatively significant amount ΔE so that theadjustment will be more than adequate to assure the power adjusted framewill be properly received even if the propagation path has deterioratedin the interim. In one implementation, the relatively significant amountΔE is at least one decibel. In the exemplary embodiment of a spreadspectrum communication system, this increase in power to mobile station30 causes less power to be available for other users who share theforward link. So the base station transmitter quickly reduces thetransmission energy for that user following the initial increase. In theexemplary embodiment, the base station increases the energy by a fixedamount ΔE holds that value for a delay period to verify that theincrease in transmission energy has been effective and then decreasesthe transmission energy in accordance with a predetermined piecewiselinear function as illustrated in FIG. 3.

[0053]FIG. 3 illustrates a plot of the transmission energy (E) againsttime. At point A the base station 50 increases the transmission energyin response to a power adjustment request from mobile station 30. Basestation 50 increases the transmission energy by an amount ΔE to point B.Base station 50 holds transmission at that transmission energy for apredetermined delay period then reduces the transmission energy at aswiftly decreasing rate for a predetermined number of frames to point C.At point C, the power control message from mobile station 30 stillindicating an excess of transmission energy, base station 50 continuesto decrease the transmission energy, however, the rate of the decreaseis less. Again, base station 50 decreases at this intermediate rate ofdecrease for a predetermined number of frames until point D. At point Dthe rate of decrease is again reduced to a final decreasing rate atwhich the transmission energy will continue to be decreased until basestation 50 reaches some minimum value or it is alerted again by anotherpower adjustment request from mobile station 30, which occurs at pointE. This power adjustment continues throughout the duration of theservice provided.

[0054] Base station 50 performs the adjustment of the transmissionenergy with knowledge that after the transmission energy has beenincreased there will be a delay before the received power controlinformation will reflect the change in the forward link transmissionpower. If the propagation channel suddenly worsens, base station 50 willreceive a series of consecutive power control requests, and there willbe a delay before the power adjustment requests are responsive to thechange in forward link transmission energy. During this delay period,base station 50 should not continue to increase the transmission energyfor each receives power adjustment request. This is the reason that thepower level is held constant for a predetermined delay period asillustrated in the period following point B of FIG. 3.

[0055] It should also be noted that errors in a mobile communicationsystem come in two types. Those that are random and those that are theresult of a change in the propagation path. In the exemplary embodiment,when base station 50 receives a power adjustment request, it increasesthe transmission power by ΔE as described previously. Then it ignoresthe power adjustment requests and retains the same increased power levelfor the delay period. In an alternative embodiment, base station 50adjusts the power in accordance with each power control message.However, smaller changes would typically be used. This minimizes theimpact of random errors.

[0056] One of the main influences that results in changes in thecharacteristics of the propagation path between mobile station 30 andbase station 50 is motion by mobile station 30 towards or away from basestation 50. Mobile station 30 may provide base station 50 withinformation indicating that the mobile station velocity is changing orit may actually provide its velocity relative to base station 50. If themobile station is simply providing an indication that its velocity ischanging, it may provide that information as a power adjustment requestsignal in anticipation of a change in the quality of the propagationpath.

[0057] In a first embodiment, mobile station 30 may sense the change invelocity by providing a sensor to operate in accordance with a signalfrom the automobile tachometer or speedometer (not shown). In analternative embodiment, mobile station 30 determines either a change inthe mobile/base station relative velocity or absolute velocity bychanges in the received signal from base station 50. Mobile station 30may detect a change in velocity or measure the absolute relativevelocity by measuring the Doppler effect on the incoming signal frombase station 50. In an alternative embodiment, base station 50 may alsodetect a change in the mobile/base station relative change in velocityor measure the absolute relative velocity by measuring the Dopplereffect on the incoming signal from mobile station 30.

[0058] The traffic signal provided by base station 50 may be accompaniedby a pilot signal in order to provide for coherent demodulation of thereceived traffic signal. Use of a pilot signal is described in U.S. Pat.Nos. 4,901,307 and 5,103,459, and mobile station 30 can alternativelysense changes in the relative velocity the Doppler shift of the pilotsignal.

[0059] In a preferred embodiment, when base station 50 knows thevelocity of mobile station 30 and will vary the value of the incrementalchange in transmission energy, ΔE, will vary in accordance with thisvelocity. The determination of the value of ΔE may be performedalgorithmically or by a lookup table in control processor 46.

[0060] If base station 50 transmits a pilot signal along with thetraffic signal, the pilot signal can be thought of as a traffic signalthat carries a predetermined bit stream known by mobile station 30.Mobile station 30 demodulates the pilot channel in pilot demodulator 45in order to get timing information to enable mobile station 30 toperform coherent demodulation of the traffic channel. Because the pilotchannel and the traffic channel are provided through similar if notidentical propagation paths, there is a strong correlation between thestrength of the received pilot signal and the strength of the receivedtraffic signal. By basing the generation of the power control signal onthe pilot channel instead of the traffic channel, the delay betweenreceiving the signal transmitted from base station 50 and generation ofthe power control signal may be reduced.

[0061] Referring to FIG. 2, pilot modulator 65 provides a pilot signalto transmitter 64 and transmitter 64 of base station 50 provides thepilot signal along with the traffic signal to antenna 52 for broadcastto mobile station 30. The transmitted signal is received at antenna 40and provided to receiver 42. Receiver 42 downconverts and amplifies thepilot signal and provides the received pilot signal to pilot demodulator45 generates a quality estimate of the demodulated pilot signal andprovides it to control processor 46. Control processor 46 generates apower control signal in accordance with the quality estimate of thedemodulated pilot signal and the operation proceeds as describedpreviously.

[0062] The previous description of the preferred embodiments is providedto enable any person skilled in the art to make or use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

We claim:
 1. A method for controlling transmission power levels,comprising: determining a velocity of a first station; using thevelocity of the first station at a second station to determine ananticipatory power adjustment; and making a transmission power levelincrease or decrease prior to receiving a signal from the first station.2. The method in accordance with claim 1 , wherein the second station isequipped with a motion sensor, and wherein the motion sensor is used todetermine the velocity of the first station.
 3. The method in accordancewith claim 1 , wherein the second station is equipped with a sensor todetermine any shift in the received signal from the first station, andwherein the shift in the received signal is used to determine thevelocity of the first station.
 4. The method in accordance with claim 1, wherein the second station is equipped with a sensor to determineselected changes in the signal received from the first station, andwherein a change in the received signal is used to determine thevelocity of the first station.
 5. The method in accordance with claim 3, wherein the second station is equipped with a sensor to determinechanges in relative velocity of the stations by measuring the Dopplershift in a received pilot signal.